Mitochondrial oxidative phosphorylation (OXPHOS) deficiencies account for a large group of heterogeneous multisystemic disorders and have also been associated to neurodegeneration and aging. The OXPHOS system is comprised of the respiratory chain (MRC) formed by enzymatic Complexes I-IV (CI to CIV) and the ATP synthase (Complex V). The RC complexes are organized into supercomplexes or respirasomes to facilitate substrate and electron channeling and minimize formation of reactive oxygen species. As a consequence of this organization, single complex biogenesis depends on other RC components and alterations in one protein-coding gene can result in combined enzyme complex defects. A better understanding of RC biogenesis is essential for elucidating the molecular basis underlying these diseases. The main objective of the proposed research is to investigate the players and mechanisms involved in RC supercomplex assembly using the yeast Saccharomyces cerevisiae and cultured human cells as research models. Our long-term goal is to attain a complete understanding of the pathways leading to respirasome biogenesis and their components as a prerequisite to the development of therapies for the management of disorders associated with RC deficiencies. We have recently reported the first description of the MRC supercomplexes biosynthetic pathway in human cells. Our data indicate that respirasome biogenesis involves a complex I assembly intermediate acting as a scaffold for the combined incorporation of complexes III and IV subunits, rather than originating from the association of preassembled individual holoenzymes. The process ends with the incorporation of complex I NADH dehydrogenase catalytic module, which leads to the respirasome activation. The central hypothesis of this proposal is that while complexes III and IV assemble either as free holoenzymes or by incorporation of free subunits into supercomplexes, the respirasomes constitute the structural units where complex I is assembled and activated, thus explaining the functional significance of the respirasomes for RC function. Three specific aims are proposed to characterize the supercomplex biosynthetic pathway and its regulation in health and disease. Aim # 1 - To refine the proposed supercomplex assembly pathway and test its universality in transformed and non-transformed cell lines Aim # 2 - To investigate the supercomplex assembly pathway in absence of one of the holoenzymes Aim # 3 - To explore the assembly of supercomplexes under cellular stress conditions that induce the expression of MRC enzyme subunit isoforms